Main group salen-based catalysts for lactide ring-opening polymerisation

Abstract

Lactide ring-opening polymerisation (LA ROP) is an efficient route to convert biomass into biodegradable polylactic acid (PLA) with high molecular weight and well controlled polymer microstructure. So far, most catalyst development for LA ROP has focused on organometallic complexes; some have displayed high catalytic activities and exquisite polymerisation control. Salen-derived complexes have shown particular success in the preparation of PLA, and the ligand can be readily modified to fine-tune the sterics and electronics of the catalytic system. Heterometallic cooperativity offers an attractive alternative approach to modify the catalyst system via incorporating a second metal, yet remains relatively underexplored. Based on half- salen and TrenSal ligands, this work investigates the influence of ligand substituents and heterometal upon the catalyst activity and control in rac-LA ROP.

Chapter 2 reports seven lithium half-salen complexes supported by sterically and electronically diverse phenolic substituents. The lithium complexes exhibit extremely high activities toward rac-LA ROP in the presence of co-initiator benzyl alcohol (BnOH) giving kobs values up to 4.46 min-1. However, the ortho-Cl/N-Me substituted complex, which features an unusual lithium phenoxyimine ladder structure, is completely inactive. The results highlight that the aggregation state can override the nature of the ligand substituents to affect the catalyst activity. Reactivity studies indicate that both coordination-insertion and activated monomer mechanisms are available depending on the absence or presence of the BnOH co-initiator, respectively generating cyclic or linear polymers.

Chapter 3 describes four new homo- and heterometallic sodium and/or aluminium TrenSal complexes for rac-LA ROP. Reactivity studies suggest that the insertion of sodium to form the heterometallic Na/Al complex alters the aluminium geometry from octahedral to tetrahedral, switching from an inactive mono-Al complex to an active and controlled heterometallic catalyst for rac-LA polymerisation in the presence of BnOH (kobs = 3.19 × 10-2 min-1). Under identical conditions, homometallic mono- and tris-Na complexes show higher catalytic activities than the heterometallic analogue albeit with poorer polymerisation control. The tris-Na complex is extremely active (kobs = 1.21 min-1) but displays unusual second-order monomer dependency on LA, which is attributed to simultaneous coordination-insertion and activated monomer mechanisms. Combing sodium with aluminium improves the polymerisation control and also prevents these two mechanisms occurring simultaneously. Overall, the heterometallic Na/Al complex combines the outstanding catalytic activity (Na) and good control (Al) of homometallic analogues.

Building upon the work in Chapter 3, Chapter 4 reports a series of novel homo- and heterobimetallic TrenSal complexes based on alkali metal (AM), magnesium and zinc. Most of the bimetallic catalysts outperform their monometallic analogues, displaying good activity and control for rac-LA ROP in the presence of BnOH (including Na/Mg, K/Mg, Na/Zn, K/Zn, Mg/Zn and Zn/Zn combinations). In contrast, the mono-Zn complex is completely inactive under the polymerisation conditions tested and the mono-AM complexes display poor polymerisation control. The choice of the AM is important, as the heterometallic complexes featuring larger potassium centres exhibit higher activity than the sodium analogues. The Zn-based heterometallic complexes are also significantly faster than the Mg-based heterometallic analogues, despite the similarity in the ionic radius and the almost identical molecular structures between relative analogues (e.g. Na/Mg and Na/Zn). Accordingly, the highest catalytic activity was observed with the K/Zn complex (kobs = 1.46 min-1). The incorporation of crown ether to the K/Zn complex leads to rearrangement of the molecular structure and slower catalytic activity towards LA ROP. This work suggests that heterometallic complexes with an AM in the outer pocket of the TrenSal ligand may improve the catalyst activity by increasing the accessibility and Lewis acidity of the monomer coordination sites, and may convert the mechanism from coordination-insertion to activated monomer.